1. Field of the Invention
The present invention relates to a magnetic bubble driving device for producing a rotating magnetic field in a magnetic bubble memory unit.
2. Description of the Prior Art
A rotating magnetic field is indispensable for the propagation, transfer, and expansion of magnetic bubble domains which correspond to binary bits `1` or `0` in a magnetic bubble memory. For details of such a rotating magnetic field, reference is made to a paper by Frantz Navratil, entitled "Generation and Fast Switching of High Frequency Rotating Fields for Bubble Memories," IEEE Transactions on Magnetics, Vol. MAG-11, No. 5, September issue, 1975, pp. 1154-1156.
As stated in the Navratil paper, a rotating magnetic field can be developed for use in a bubble driving device by supplying periodic coil currents which are 1/4-period out of phase with each other, respectively, to a couple of drive coils wound orthogonally to each other and cylindrically around a bubble memory unit comprising at least one memory chip. Sinusoidal or triangular shaped waveforms, for example, may be used in such an arrangement.
Generally, the coils are driven by sinusoidal currents produced by LC resonance circuits, each comprising a coupling of a drive coil and a capacitor connected in parallel. Since each resonance circuit typically employs a current switch in series with the drive coil for the start-stop operation of the coil current, however, a relatively large amount of attenuation of the sinusoidal coil current occurs due to resistance loss. In order to avoid such deterioration in the coil current waveform, an excitation switch is added for each resonance circuit to provide loss compensation and to boost the voltage across the capacitor at each half period to approximately that of the positive or negative initial voltage. Consequently, two or more power supply sources are necessary to maintain these positive or negative initial voltage levels. Furthermore, in order that the resonance frequency of each LC circuit is in synchronism with the operating frequency of the excitation switch, the capacitance of the capacitor in each resonance circuit must be suitably adjusted. Also, troublesome timing control is required after the power supply sources are turned on or at the time of the driving initiation of the coil currents, in order to set the potential across each capacitor to a given initial value. Such requirements complicate the circuit construction of each resonance circuit. Despite these disadvantages, however, such resonant circuits have been the predominant means used for driving a rotating magnetic field in a bubble memory, since conventional bubble memories have been developed to adapt to such sinusoidal driving currents.
Recently, non-resonance circuits, each of which has a simple structure comprised of switches and diodes, have been used to produce rotating magnetic fields by means of triangular wave drive coil currents. Such circuits allow not only easy start-stop control of the coil currents but also allow the use of a single power supply source. For these reasons, it is preferable to adapt bubble memory structures to use driving devices powered by triangular coil currents. The use of this type of driving device having no resonance capacitors is especially desirable when the device is to be fabricated on a substrate by IC (integration circuit) technology for size minimization.
Theoretically, in such triangular coil current driving systems, each coil current has an ideal triangular waveform, i.e. the current rises linearly from 0 to I.sub.L during the first 1/4 period and then falls linearly from I.sub.L to 0 during the next 1/4 period. Thus, one coil current with such an ideal triangular waveform when combined with another one shifted 1/4 period out of phase from the first forms a diamond-shaped Lissajous' figure. If the amplitude of a circle internally contacting the resulting diamond-shaped Lissajous' figure corresponds to I.sub.R, I.sub.R is given as follows: ##EQU1##
Therefore, with such a triangular waveform system, even if each coil current follows an ideal triangular waveform, the amplitude in any direction of the Lissajous' figure lies between I.sub.R (minimum amplitude) and I.sub.L (maximum amplitude). Therefore, driving efficiency, defined as the ratio of I.sub.R to I.sub.L, using such ideal triangular coil currents cannot be enhanced to a value greater than l/.sqroot.2=0.7 as seen from equation (1). As a result, power consumption and induction noises affecting the sense system are greater than with sinusoidal coil currents.
In conventional driving devices using triangular waveform drive currents, the rise time T.sub.r is not always the same as the fall time T.sub.f. Particularly when the rise time T.sub.r (=T.sub.o /4) is not sufficiently smaller than the time constant L/R, the fall time T.sub.f will be less than the rise time T.sub.r ; where T.sub.o represents the driving period (the inverse of the driving frequency), L the inductance of the driving coil, and R the resistance of the coil. Consequently, the driving efficiency drops to 0.6, or less, resulting in additional increases in amplitude variation, sense noise, and power consumption.
Accordingly, one object of the present invention is to provide a magnetic bubble driving device which is free from the above-mentioned and other disadvantages of conventional systems.